1. Field of the Invention
This invention relates to aluminum alloy extruded articles or extruded profiles (materials shaped into a continuous form by extrusion) adapted to undergo anodic oxidation prior to use, and more particularly to extruded articles made of age-hardening aluminum alloy to be subjected to an integral coloring in gray and to be used in such interior and exterior building materials as sashes, curtain walls, and gates or in frames of such electric appliances as audio devices which by nature need ornamentality and to a method for the production thereof.
2. Description of the Prior Art
In the building grade extruded articles of aluminum alloy, those of the JIS (Japanese Industrial Standard) A6000 type, particularly the 6063 alloy, have been used prevalently because they exhibit ideal resistance to corrosion, possess high strength, and excel in malleability for extrusion or extrudability. These alloys generally undergo an anodizing treatment prior to use and they come in a silver color inherent in aluminum. For the sake of acquiring further exalted ornamentality, they are subjected to secondary electrolytic coloring in an electrolytic solution containing Ni, Co, Sn, or the like. The color consequently imparted thereto, however, is limited to one and the same bronze type color. There is much need also for building materials which come in varying colors other than the color mentioned above. In recent years, demands have been mounting particularly for the building materials which are in an achromatic or neutral gray color possessed of calm tone and substantial depth.
As alloys which are made to educe a gray color or to be naturally colored in gray by the anodizing treatment thereof, the Al--Fe alloys, the Al--Si alloys, or the Al--Mg--Si alloys adapted to harden by aging owing to the incorporation of magnesium therein have been known in the art. In the case of an alloy which contains Fe as a coloring element, for example, when it has its Fe content increased for the sake of darkening the color to be educed, the increased Fe content entails the disadvantage of degrading strength, giving rise to coarse and unevenly distributed grains of an Al--Fe compound in the matrix of alloy, and consequently damaging the uniformity of coloration of the alloy during the course of the anodic oxidation.
Then, in the case of an alloy which contains silicon as a coloring element, when it is heat-treated and extruded under ordinary conditions, the amount of fine silicon particles subsequently precipitated therein and expected to contribute to the integral coloring of the aluminum alloy is so small as to render desired darkening of color difficult. This darkening of color could be attained by increasing the thickness of an anodic oxide film of the alloy. This method, however, is at a disadvantage in sacrificing economy owing to consumption of unduly large electric power for the treatment of anodic oxidation and suffering proportionate growth of yellowish and reddish tint due to the increase in the thickness of the anodic oxide film and inevitably acquiring a color tone which is different from the achromatic color aimed primarily at by the method. Particularly, in the case of an age-hardening type alloy which incorporates magnesium therein, the precipitation of Mg.sub.2 Si which occurs during the course of the aging treatment results in consuming silicon as a coloring element and rendering it difficult to accomplish uniform dispersion of fine precipitated silicon particles in the alloy matrix. This alloy, therefore, incurs extreme difficulty in acquiring desired color tone and color darkness, let alone strength, stability with good reproducibility owing to the interaction metallurgical factors as mentioned above.